The structure of a transformer is prescribed by IEC (International Electrotechnical Communication) Standards Pub. 950, etc. That is, these standards provide that at least three insulating layers be formed between primary and secondary windings in a winding, in which an enamel film which covers a conductor of a winding be not authorized as an insulating layer, or that the thickness of an insulating layer be 0.4 mm or more. The standards also provide that the creeping distance between the primary and secondary windings, which varies depending on the applied voltage, be 5 mm or more, that the transformer withstand a voltage of 3,000 V applied between the primary and secondary sides for a minute or more, and the like.
According to such the standards, as a currently prevailing transformer has a structure such as the one illustrated in a cross-section of FIG. 2. In the structure, an enameled primary winding 4 is wound around a bobbin 2 on a ferrite core 1 in a manner such that insulating barriers 3 for securing the creeping distance are arranged individually on the opposite sides of the peripheral surface of the bobbin. An insulating tape 5 is wound for at least three turns on the primary winding 4, additional insulating barriers 3 for securing the creeping distance are arranged on the insulating tape, and an enameled secondary winding 6 is then wound around the insulating tape.
Recently, a transformer having a structure which includes neither the insulating barriers 3 nor the insulating tape layer 5, as shown in FIG. 1, has started to be used in place of the transformer having the structure shown in the cross-section of FIG. 2. The transformer shown in FIG. 1 has an advantage over the one having the structure shown in FIG. 2 in being able to be reduced in overall size and dispense with the winding operation for the insulating tape.
In manufacturing the transformer shown in FIG. 1, it is necessary, in consideration of the aforesaid IEC standards, that at least three insulating layers 4b (6b), 4c (6c), and 4d (6d) are formed on the outer peripheral surface on one or both of conductors 4a (6a) of the primary winding 4 and the secondary winding 6 used.
As such a winding, a winding in which an insulating tape is first wound around a conductor to form a first insulating layer thereon, and is further wound to form second and third insulating layers in succession, so as to form three insulating layers that are separable from one another, is known. Further, a winding in which a conductor enameled with polyurethane is successively extrusion-coated with a fluororesin, whereby extrusion-coating layers composed of three layers structure in all are formed for use as insulating layers, is known (JU-A-3-56112 ("JU-A" means unexamined published Japanese Utility Model application).
In the above-mentioned case of winding an insulating tape, however, because winding the tape is an unavoidable operation, the efficiency of production is extremely low, and thus the cost of the electrical wire is conspicuously increased.
In the above-mentioned case of extrusion of a fluororesin, since the insulating layer is made of the fluororesin, there is the advantage of good heat resistance and high-frequency characteristic. On the other hand, because of the high cost of the resin and the property that when it is pulled at a high shearing speed, the state of the external appearance is deteriorated, it is difficult to increase the production speed, and like the insulating tape, the cost of the electric wire becomes high. Further, in this case of the insulating layer, there is a problem that, since the insulating layer cannot be removed by dipping in a solder bath, the insulating layer on the terminal has to be removed using less reliable mechanical means, and further the wire must be soldered or solderless-connected, when the terminal is worked for the insulated wire to be connected, for example, to a terminal.
On the other hand, a multilayer insulated wire is put to practical use, wherein multilayer extrusion-insulating layers are formed from a mixture of a polyethylene terephthalate as a base resin with an ionomer prepared by converting part of carboxyl groups of an ethylene/methacrylic acid copolymer to metal salts, and wherein the uppermost covering layer among the insulating layers is made of a polyamide (nylon). This multilayer insulated wire is excellent in cost of electrical wire (nonexpensive materials and high producibility), solderability (to make possible direct connection between an insulated wire and a terminal), and coilability (that means that, in winding the insulated wire around a bobbin, the insulating layer is not broken to damage the electrical properties of the coil, when, for example, parts of the insulated wire are rubbed with each other or the insulated wire is rubbed with a guide nozzle) (U.S. Pat. No. 5,606,152, and JP-A-6-223634 ("JP-A" means unexamined published Japanese patent application)).
Further, to improve heat resistance, the inventors proposed an insulated wire whose base resin is changed from the above polyethylene terephthalate to polycyclohexanedimethylene terephthalate (PCT).
The heat resistance of these multilayer insulated wires is acceptable to heat-resistance Class E in the test method in conformity to Annex U (Insulated wires) of Item 2.9.4.4 and Annex C (Transformers) of Item 1.5.3 of the IEC 950-standards, and there is no problem on the heat resistance. However, in recent years, the frequency used in transformers in circuits is made into higher frequencies, and in order to meet the higher required level from now on, a further improvement in electrical properties at higher frequencies is demanded.
Further, in a multilayer insulated wire having a self-bonding layer on an extrusion-coating insulating layer, the self-bonding layer is sometimes scraped from the adhered parts in the vicinity between wires by corona under high voltage and high frequencies, and therefore an improvement in physical properties under high voltage and high frequencies is desired similarly to the above.
To solve such problems, an object of the present invention is to provide a multilayer insulated wire that is excellent in solderability, high-frequency characteristic, prevention of scraping-off of an insulating-coating under high-voltage and high-frequency, and coilability, and that is favorably suitable for industrial production.
Further, another object of the present invention is to provide a transformer having excellent electrical properties and high reliability, wherein, when it is used at high frequencies, such problems as lowering of the electric properties, scraping of a wire by corona, and the like, do not occur, and wherein such an insulated wire excellent in solderability, high-frequency characteristic, and coilability is wound.
Other and further objects, features, and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.